Democrats have been pounding Mitt Romney relentlessly for months, issuing Web videos, press releases, and a smattering of paid ads belittling, sometimes with humor, the Republican presidential candidate as a flip-flopping shape-shifter and cold-blooded former corporate raider. With Romney sinking and Newt Gingrich rising in recent polls, the Democrats believe their efforts are hobbling the candidate they evidently fear most in a general election.

None of the other Republicans has attracted anywhere near the attention Romney has from the Democratic National Committee and allied organizations.

The steady drip-drip of the partisan assaults has gone on for months. It started when Romney was the default front-runner and continued as one flawed GOP candidate after another challenged or eclipsed him briefly in polls before crashing into single digits. Consistently in the polling, Romney has been unable to exceed 25 percent, which has allowed more conservative candidates to muster their challenges.

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“It’s definitely taking a toll,’’ said Bill Burton, a former spokesman for President Obama now with Priorities USA Action, a pro-Obama so-called super political action committee that has aired some paid broadcast ads and more on the Internet at a cost of more than $300,000. To generate news coverage, the group has also issued a series of memos attacking Romney’s shifting positions on issues.

The group produced a Web video knocking Romney’s policies as beneficial to Wall Street and detrimental to the rest of the country. As of yesterday, the video had scored more than 300,000 Internet hits.

While many voters are sizing up the candidates based on the dozen televised debates, the dearth of advertising has allowed the outside Democratic groups to have an impact, Burton said.

Romney led Gingrich, usually by wide margins, in national polls for months until November, when Gingrich began to shoot past him. Gingrich has won nine of the last 10 polls; opened up a lead in Iowa, the first caucus state; cut Romney’s big advantage in New Hampshire, the first primary state; and bolted well ahead of Romney in South Carolina and Florida.

Mike Murphy, who was Romney’s chief strategist in 2002 but has no formal role in his presidential campaign, said the Democrats’ attacks are so intense that “it’s like they have 200 guys in Chicago working’’ on it around the clock.

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“It’s a clear indication the Obama people would rather face any of the other Republicans than Romney,’’ Murphy said. “It’s a two-pronged strategy: They either hurt him enough to cost him the nomination and the threat goes away, or he is damaged as the nominee in the general election.’’

Responding to a Globe inquiry, Romney campaign spokeswoman Andrea Saul said: “President Obama has presided over an unprecedented 34 straight months of 8 percent or higher unemployment, so it’s no wonder his campaign is desperate to distract from his failed economic record. It is clear the last thing the White House wants to do is face Mitt Romney and be forced to defend three years of high unemployment and runaway spending. Rather than focusing on job creation and helping the middle class, President Obama and his campaign are focused on attacking Mitt Romney.’’

Romney told the Washington Post yesterday that Obama’s policies will be on voters’ minds next year, not “class warfare or demonization of an individual or a percentage of Americans.’’

“This is a president who talked about bringing America together,’’ Romney said. “Is he going to run a campaign based on tearing America apart?’’

The attacks on Romney, for the most part, have not been coming from the Obama reelection campaign but from the Democratic National Committee; Priorities USA Action; and Americans United for Change, a labor- and liberal-backed advocacy group.

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The DNC Web videos have zinged Romney, one day challenging him to issue his tax returns and list of fund-raising bundlers, another day calling him a career politician, though he has served fewer years in public office than Obama and all of Romney’s Republican rivals.

The DNC has established a website, MittvMitt.com, featuring the Democrats’ most extensive attack, a four-minute broadside that is an extended version of a 30-second spot the DNC aired in five battleground states and Washington, D.C.

The longer version, which had more than 282,000 hits on YouTube as of yesterday, uses Romney’s words to portray him as a flip-flopper on 11 issues, including gun control, the federal stimulus program, Ronald Reagan, and global warming. Interspersed are critical or amusing comments by television pundits, news anchors, and late-night television hosts.

Ben Clarke, senior vice president of Luntz Global, a Republican-oriented polling firm, tested the four-minute DNC video on a focus group of 28 swing voters in Orlando, Fla.

“We’re finding that attacks like this have been resonating with these voters,’’ Clarke said. “At this point, Mitt Romney needs a Ronald Reagan moment. He needs to grab the microphone and say with palpable passion why he is the right candidate.’’

Meddling in the opposing party’s primary has precedent. During the 2010 Nevada Republican primary campaign, Senate majority leader Harry Reid repeatedly attacked Sue Lowden, considered his most serious potential challenger. The effort, combined with Lowden’s gaffes, opened the door to victory for Tea Party favorite Sharron Angle, whom Reid defeated.

In 2002 the campaign of Democrat Gray Davis, vulnerable in his bid for reelection as California’s governor, spent about $7 million during the GOP primary on negative ads aimed at former Los Angeles mayor Richard Riordan, a moderate Republican, contributing to his defeat at the hands of the much more conservative Bill Simon. Davis went on to defeat Simon.